1. Technical Field
The present invention relates to a transmission system for transmitting signals, and a system startup method for starting the transmission system. In particular, the present invention relates to a transmission system for transmitting optical signals, and a system startup method for starting the transmission system.
2. Description of the Related Art
In general, in transmission systems adapted to transmit signals, a plurality of apparatuses are connected in series or in parallel. The plurality of apparatuses often have performance different from each other. Accordingly, the time from when these have been switched on until when the output levels of the optical signals outputted from the apparatuses reach a predetermined target level value (hereinafter referred to as startup time) is not uniform due to the response characteristics of devices built into each apparatus.
In apparatuses that are generally applied to transmission systems, output control is executed faster at startup time than at regular control time so that the target output level is attained as quickly as possible during signal conduction from the shutdown. Then, after the output level is settled to the target level value (after startup time), a switchover to low-speed output control is accomplished to move to normal operation.
FIG. 6 is a diagram showing an example of a related transmission system.
In the related transmission system shown in FIG. 6, an apparatus 901 and an apparatus 902 are connected in series. The apparatuses 901 and 902 are transmission apparatuses for transmitting optical signals, and optical signals outputted from the apparatus 901 (upstream) are entered into the apparatus 902 (downstream) located at the subsequent stage of the apparatus 901.
If the startup time of the apparatus 902 is longer than the startup time of the apparatus 901, two problems as described below may arise.
(1) The output level of an optical signal outputted from the apparatus 901 continues changing until the output level reaches an output target level value, and, as a result, the input level of an optical signal entered into the apparatus 902 also varies. In this case, the apparatus 902 cannot suppress a fluctuation in the input level. As a result, unnecessary output level fluctuation occurs in the apparatus 902. If other apparatuses are connected downstream from the apparatus 902, when the fluctuation is propagated downstream, further downstream the signal goes, the more unstable the optical level becomes. Therefore, the more the number of connected nodes, the longer the period of time the output level converges to the target level value would be.
(2) The input level of an optical signal entered into the apparatus 902 may be lower than a specified value until the output level of the optical signal outputted from the apparatus 901 reaches the target level value. Therefore, an alarm indicating that the level is low may be issued.
Hence, a technique has been considered in which the control of its own station is entered in a standby (stop) state until the fluctuation of a station located upstream from its own station is completed (e.g., Japanese Patent Laid-open Application Publication No. 2006-033542).
In addition, a technique has also been considered in which, in order to prevent an incorrect alarm from being notified, an alarm is masked until a predetermined time has elapsed from when an optical switch started switchover operation (e.g., Japanese Patent Laid-open Application Publication No. 2003-209864).
However, in the technique described in Japanese Patent Laid-open Application Publication No. 2006-033542, there is a problem that the startup time of the system becomes long because the control of its own station is entered in a standby (stop) state.
In addition, in the technique described in Japanese Patent Laid-open Application Publication No. 2003-209864, there is a problem that a signal to mask an alarm has to be generated. There is another problem that the time period over which an alarm is masked has to be set in advance.